1. Field of the Invention
The present invention relates to conductive ink to be applied to an electrode pattern disposed in the inside and outside of a ceramic multi-layer substrate on which a semiconductor LSI, chips and the like are installed and wired.
2. Description of the Related Arts
In recent years, the general tendency is to manufacture compact and light semiconductor LSI and chips and thus, there are growing demands for the manufacture of compact and light wiring substrates on which they are mounted. In such a situation, a great importance is attached to ceramic multi-layer substrates in electronic component-manufacturing trade because semiconductor LSI and chips can be wired in a high density thereon and can be formed to be thin. Conductive materials serving as an electrode to be mounted on the ceramic multi-layer substrate include a paste compound comprising an organic media in which a conductive metal, inorganic oxide and glass powders are dispersed. Owing to the development of the multi-layer substrate made of glass/ceramic to be sintered at a low temperature in recent years, gold, silver, copper, palladium or mixtures thereof have been used as materials of the electrode. Gold, silver, copper and palladium have a lower resistivity than tungsten or molybdenum conventionally used. Further, equipments for manufacturing the multi-layer substrate made of the former is safer than those for manufacturing the multi-layer substrate made of the latter and the multi-layer substrate made of the former can be manufactured at a less expensive cost than multi-layer substrate made of the latter.
Gold, silver, and palladium, which are noble metals, are expensive and the prices thereof fluctuate greatly. Thus, recently, base metals are used more frequently than noble metals because the former is less expensive than the latter and the prices of the former do not fluctuate so greatly as the latter. Above all, copper is most favorably used as a material of an electrode because the resistivity thereof is low and solder-wettability thereof is superior.
According to a method, copper is used as the material of the multi-layer substrate to be sintered at a low temperature, and copper is used as the material of the uppermost and inner layers thereof. This method is preferable in respect to resistance, solder-wettability, and cost. But it is necessary to sinter copper in a neutral atmosphere consisting of such gases as nitrogen and thus it is difficult to prepare a multi-layer substrate of copper. Generally, in composing an electrode of copper, a wiring pattern is formed on a substrate by screen-printing copper paste. After the copper paste is dried, the copper paste is sintered at a temperature (850.degree.-950.degree. C.) less than the melting point of copper in a nitrogen atmosphere in which the oxygen partial pressure is controlled, so that copper is not oxidized and the organic component of the conductive paste can be burned sufficiently burned. In layering a plurality of substrates one on the other, insulating layers are printed and sintered in a similar condition. But it is difficult to adjust the atmosphere in the sintering process to an appropriate oxygen partial pressure. In layering a plurality of substrates one on the other, it is necessary to sinter each copper paste repeatedly after each copper paste is printed and thus lead time is long. As a result, the cost for installing equipments is high (Japanese Patent Laid-Open Publication No. 57-53321). In order to overcome this disadvantage, a method for manufacturing a ceramic multi-layer substrate is disclosed in Japanese Patent No. 5-1774496. This method comprises a binder-burn-out process, a reduction process, and a firing process. That is, a multi-layer substrate is manufactured with CuO serving as a starting material of a conductor. In the binder-burn-out process, heat treatment is performed in an oxygen atmosphere suitable at a temperature high enough to thermally decompose the organic binder contained in the multi-layer substrate. In the reduction process, CuO is reduced to Cu. In the firing process, the substrate is sintered. In this manner, the atmosphere can be controlled easily in the firing process and a fine sintered substrate can be obtained.
In the firing process, the ceramic multi-layer substrate shrinks due to the sintering. The degree of shrinkage which occurs due to the sintering is different from each other according to the material of the substrate, the composition of a green sheet, and the lot number of the powder. This method has the following problems. The first problem is that if the shrinkage error of the material of the substrate is great, a wiring pattern of the uppermost layer cannot be connected with electrodes of the inner layers due to a dimension error therebetween because as described above, in the manufacture of the multi-layer substrate, the wiring of the uppermost layer is formed after metals of the inner wirings are fired. As a result, it is necessary to form a land having a large area, more than is required, on the electrode of the uppermost layer so as tolerate a shrinkage error which will be made by the shrinkage. Thus, it is difficult to form on the ceramic multi-layer substrate a circuit requiring wiring of a high density. Therefore, a plurality of screen plates for use in the wiring of the uppermost layer is prepared in accordance with the degrees of shrinkage errors so that the screen plates are selectively used according to the degree of shrinkage of the substrate. This method requires many screen plates and is hence uneconomical.
Co-firing the wiring of the uppermost layer and that of the inner layer eliminates the need for the formation of the large land. But this method does not eliminate the shrinkage error of the substrate itself. In cream solder and printing in mounting parts on the substrate, there is a case in which the shrinkage error may not allow printing to be performed on a necessary portion and in addition, parts are not mounted on a predetermined mounting position.
Secondly, in the multi-layer substrate to be formed by layering green sheets one on the other, the degree of shrinkage of the multi-layer substrate in a widthwise direction thereof is different from that in a lengthwise direction thereof, depending on a film-forming direction of the green sheet. This is an obstacle in manufacturing the ceramic multi-layer substrate.
In order to reduce the shrinkage error as greatly as possible, it is necessary to control the composition of the multi-layer substrate and that of the green sheet, the difference in the lot number of the powder or lamination condition such as pressure and temperature in manufacturing the multi-layer substrate. Generally, it is said that the error in the degree of shrinkage is approximately .+-.0.5%.
This is a problem of not only the multi-layer substrate ceramic, but also of glass/ceramic which is sintered. In order to solve this problem, the following method is proposed in Japanese Patent Laid-Open Publication No. 5-102666. According to this method, green sheets, comprising glass/ceramic to be sintered at a low temperature, on which electrode patterns have been formed are laminated in a desired number one on the other. Then, green sheets disposed at the upper and/or lower end of the layered green sheets are covered with green sheets comprising inorganic compounds not sintered at the firing temperature of the material of the glass/ceramic substrate to be sintered at a low temperature. Thereafter, the layered green sheets are fired. Then, the inorganic compounds are removed from the layered green sheets. In this manner, the material of the substrate is sintered vertically and does not shrink horizontally and thus the above-described problems can be solved.
This method for forming a substrate has, however, the following problems: An electrode comprising such a conventional paste compound as described above has a porous structure because in firing the substrate, the substrate shrinks vertically. The electrode having a porous structure does not adhere to the substrate strongly. Consequently, there is a possibility that electrode is removed from the substrate or easily oxidized because the electrode is exposed to outside air in a great area. It is conceivable to carry out a method of increasing the mixing ratio of a conductive material or reduce the diameter thereof so as to form an electrode having a fine structure. According to this method, the conductive material starts to sinter earlier than the substrate in firing the glass/ceramic multi-layer substrate. Therefore, the substrate is incapable of preventing the conductive material from sintering. As a result, the substrate is crackled at a portion in the vicinity of the electrode.
In order to form the glass/ceramic substrate which does not shrink horizontally, it is necessary to use a conductive paste suitable for the substrate.